How to change Follow Path Speed
If you're asking about moving the object the full length of the path at a slower pace, read on...  If you're trying to vary, stop, or even make the object go backwards and forwards, that's a bit more complicated but can be added to explain this.  Just ask.

For the full path length at a slower pace -

Go into the path's properties (at right) and set the "Frames" value to twice what you had before.  Make sure your animation frame range is large enough to accomodate that number of frames.  

In the Evaluation slot it's a good idea to delete the maximum frame number you have there already.  (set the timeline cursor to that frame number, hover over the slot and press ALT-i)

Now set the timeline cursor to the same frame number as you entered into the "Frames" slot.

Enter that number into the Evaluation slot and hover over it with the mouse and press i to keyframe it.

You've then doubled the number of frames (time) that your object has to complete the journey.